Blood glucose monitoring is a necessary tool for achieving glycemic control in diabetics. By permitting the patient to recognize when he is in a state of either hypo- or hyperglycemia, this monitoring enables the patient to appropriately intervene to maintain his health.
Hypoglycemia, the condition of having too little blood glucose, in severe form referred to as insulin shock, may lead to unconsciousness and death. As this may occur while the patient is asleep, monitoring methods that require action on the patient""s part leave the patient vulnerable during his sleep. Frequent episodes of hyperglycemia lead to chronic diabetes complications, such as blindness, kidney failure and/or limb amputations.
Currently available methods of glucose monitoring require the patient to obtain a blood sample. Testing a blood specimen permits detection of both hypoglycemia and hyperglycemia. Unfortunately, the xe2x80x9cfinger stickxe2x80x9d generally required for specimen collection is painful, so this type of testing is unpopular with patients and sometimes avoided. Newer, less painful methods are somewhat cumbersome.
Accordingly, continuous in vivo monitoring would yield great advantages by allowing prompter patient intervention. Many efforts to achieve this goal have been made over the years.
Generally, continuous in vivo monitoring is done with a sensor that produces an electrical current that is proportional to the blood or subcutaneous tissue glucose level. This is done by creating a reaction between immobilized glucose oxidase mixed with Bovine or Human Serum Albumin and glucose, to form gluconic acid and hydrogen peroxide. The hydrogen peroxide is oxidized at the platinum-indicating electrode 20 or anode surface, thereby freeing electrons that create a current and flow into the anode. Alternatively, or additionally, a current that is proportional to the dissolved oxygen level, which is decreased by the reaction in which glucose is oxidized, can be monitored. U.S. Pat. No. 5,165,407 (""407) and U.S. Pat. No. 5,711,861 (""861), which are both hereby incorporated by reference as if fully set forth herein, disclose in vivo devices for sensing glucose levels.
There are, however, a number of additional technical problems that must be addressed in the design of a glucose sensor. First, both dissolved oxygen and glucose are necessary to create the reaction that produces H2O2. Because dissolved oxygen is generally less abundant than glucose, the amount of H2O2 produced is primarily responsive to the concentration of dissolved oxygen, unless steps are taken to avoid this outcome. The region of linear response to glucose concentration may be increased by a membrane that restricts the passage of glucose molecules while permitting the relatively unrestricted passage of oxygen molecules. As a result, there is adequate dissolved oxygen to permit full registration of even relatively high concentrations of glucose. Permselective membranes currently in use have proven quite problematic, generally displaying poor performance.
One family of substances that has been suggested for use as a permeable selective (permselective) membrane are polyurethane/polyurea compositions containing silicone (taught, intra alia, in U.S. Pat. No. 5,882,494). Compounds from this family are discussed in some detail in U.S. Pat. No. 5,428,123, entitled COPOLYMER AND NON-POROUS SEMI-PERMEABLE MEMBRANE THEREOF AND ITS USE FOR PERMEATING MOLECULES OF PREDETERMINED MOLECULAR WEIGHT RANGE, which is hereby incorporated by reference as if fully set forth herein. As a term of art, the substances described and claimed in this patent are referred to as barrier breathing film (BBF). One characteristic of these substances is that they comprise a biocompatible, hydrophilic, segmented block polyurethane copolymer having a number average molecular weight of about 5,000 to 150,000, comprising about 5 to 45 wt % of at least one hard segment, and about 95 to 55 wt % of at least one soft segments comprising at least one hydrophilic, hydrophobic or amphipathic oligomer selected from the group consisting of aliphatic polyols, aliphatic and aromatic polyamines and mixtures thereof.
Another problem encountered is the presence in the interstitial fluid (ISF) of electroactive compounds other than H2O2. For example, the common pain reliever acetaminophen may be present in a patient""s body and is capable of creating current flow into the anode. To prevent this interfering signal it has been suggested by Pankaj Vadgama et al. in U.S. Pat. No. 4,832,797, entitled ENZYME ELECTRODE AND MEMBRANE, to use a membrane of a sulphonated polyethersulphone (SPES) placed directly over the indicating electrode sensing surface. This layer generally prevents the passage of compounds larger than H2O2.
A review of the art reveals that it appears to have not yet been suggested to use a layer of BBF over a layer of SPES for both selectively passing oxygen and glucose and also filtering out interferents such as acetaminophen. This is not accidental as the general industry understanding has been that it is not possible to coat SPES with BBF due to the fact that the solvents available for creating a solution of BBF (typically dimethyl acetamide [DMAC] or related solvents) are the same ones that dissolve SPES. Accordingly, there has been a long felt need in the industry for a two membrane combination that can both selectively filter oxygen and glucose and can also filter out electro active interferents, despite the industry knowledge of both BBF and SPES. Moreover, it has been difficult to get the SPES to adhere strongly enough to the platinum-iridium surfaces so that a uniform coat of SPES is formed over the exposed platinum-iridium surface(s).
Another problem encountered in this type of sensor is the tendency for the sensitivity of the sensing surface to drift over time. One cause of this drift is growth of scar tissue about a sensor.
Another challenge for those designing needle sensors in general is that of potential breakage. It is unacceptable to leave a piece of a needle sensor within the human body. Accordingly, needle sensors that can be flexed many times (upwards of 1,000) are highly desirable compared with those that risk breakage with 30 or fewer flexures.
In a first separate aspect the present invention is an insertable analyte needle sensor, comprising a set of fine wires positioned together and a dielectric material covering a substantial portion of the fine wires but defining an opening filled with at least one partially permeable membrane.
In a second separate aspect the present invention is a method of producing an analyte sensor, comprising the steps of providing a conductive wire having an exposed surface and gas plasma treating the exposed surface of the conductive wire to remove oxidation. A SPES membrane is then applied over the platinum-iridium wire prior to the reformation of a layer of oxidation.
In a third separate aspect the present invention is a method of producing an analyte sensor in which, comprising the steps of providing a conductive wire having an exposed surface, applying a SPES membrane over the exposed surface of the conductive wire, and coating the SPES membrane with a solution of silane.
The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the preferred embodiment(s), taken in conjunction with the accompanying drawings.